Control of rhythmic movements in animals is mediated by specific neuronal interactions that occur in the spinal cord and brain of vertebrates and in the segmental ganglia and cephalic ganglia of invertebrates. The research proposed here is designed to elucidate mechanisms underlying the control of such movements through the investigation of swimming activity in an invertebrate model, the medicinal leech. The specific aims are to: 1) discover mechanisms by which brief stimulation of "trigger" neurons in the subesophageal ganglion elicits prolonged rhythmic motor output in midbody neurons; 2) describe the mechanisms by which inhibitory neurons in the subesophageal ganglion inhibit such motor output; and 3) determine the roles of the cephalic neurons for the initiation of locomotion in nearly intact animals. Experiments will be carried out on the ventral nerve cord and cephalic ganglia of the leech, Hirudo medicinalis. Standard physiological and morphological techniques will be employed to identify neurons, to describe neuronal circuits, and to evaluate the role of these circuits during the initiation of leech swimming movements. The long-range objective for this research project is to explain the physiological mechanisms by which the nervous system generates and controls movements in animals. The approach taken here is to study intensively the relatively simple movements of one favorable invertebrate model system. Our present models for the mechanism underlying neuronal interactions in all animals are derived largely from research on invertebrate system. The discoveries made during this investigation should likewise contribute importantly to an understanding of the neuronal control of animal movements and, by extension, to the treatment of neurological diseases.